Many women are unfortunate enough to require long term heparin prophylaxis in pregnancy. Standard heparin doesn't cross the placenta or pose a threat to the fetus, long term use of heparin frequently has adverse effects for the mother such as bleeding, osteoporosis & thrombocytopenia. A new low molecular weight heparin (LMWH) has been shown to be effective in preventing deep venous thrombosis & appears safe in pregnancy as it doesn't cross the placenta. The study will elucidate whether there is adverse effects from long term LMWH use in pregnancy.